Fixed two SQLiteC++ Coverity Issues in Database::execAndGet() and Database::tableExists()

- adding a (void) cast in front of query.executeStep()
- adding comments explaining how errors are handled in this thow methods

Thank you to Donald Jones and Mark Pashley of Ubiquisys for reporting this to me.
Fixes #1: answer the question about "Multithreading/Monothreading" an thread-safety

SQLite supports three mode of thread safety, as describe in "SQLite And Multiple Threads" :
see http://www.sqlite.org/threadsafe.html

This SQLiteC++ wrapper does not add any lock (no mutexes) nor any other thread-safety mecanism
above the SQLite library itself, by design, for lightness and speed.

Thus, SQLiteC++ naturally supports the "Multi Thread" mode of SQLite ;
"In this mode, SQLite can be safely used by multiple threads
provided that no single database connection is used simultaneously in two or more threads."

But SQLiteC++ does not support the fully thread-safe "Serialized" mode of SQLite,
because of the way it shares the underling SQLite precompiled statement
in a custom shared pointer (See the inner class "Statement::Ptr").